Solar cells are devices that have characteristics that enable them to convert the energy of sunlight into electric energy. The aim of research often is to achieve solar cell designs that are suitable for inexpensive commercial production while providing acceptably high energy conversion efficiencies.
A conventional thin film solar cell is composed of a stacking of thin layers on a rigid or flexible substrate, and the thin layers form one or more junctions that absorb light and convert it into electricity. Briefly, a typical thin film PV device such as a thin film solar cell may include a glass, metal, or polymer substrate, a back contact, an absorber, a window layer, a front contact or low resistivity layer, and a top protective layer (e.g., a glass substrate) or a similar arrangement of thin film layers. Presently, many thin film solar cells are fabricated with an absorber or absorber layer formed of copper indium diselenide (“CIS”) or copper indium gallium diselenide (“CIGS”) because an absorber formed of either material has a high optical absorption coefficient and suitable optical and electrical characteristics. With regard to CIS and CIGS solar cells, work continues to provide better methods of producing a CIS or CIGS thin film layer that is of proper composition and structure to allow charges generated by received sunlight (i.e., electrons and holes) to exist long enough in the CIS and CIGS layer of the device so that they can be separated and collected at the front and back contacts to provide higher conversion efficiency.
Commercial production of solar cells includes growth of thin films including a CIS or CIGS absorber using a variety of processes. A coevaporation process may be used to produce a thin film by concurrently evaporating copper (Cu), indium (In), gallium (Ga), and selenium (Se) from elemental sources. Even with precise control over evaporation rates, it has proven difficult to obtain a homogenous thin film with a desired roughness and uniform thickness. In other commercial production lines, selenization from selenium vapor is used to form the CIS or CIGS absorber for a solar cell. In a typical process, a substrate is provided that is a soda lime glass coated with a thin film of molybdenum (Mo), as the back contact of the solar cell. Cu and In,Ga layers are sequentially deposited on the substrate by a vapor deposition process such as sputtering. The different layers are thermally selenized in an H2Se or Se-containing atmosphere and then converted into a CIS or CIGS thin film. An advantage of this process compared with the coevaporation process is that large area depositions of CIS or CIGS films can be produced commercially.
Researchers studying techniques for fabricating higher efficiency solar cells designed an improved method of forming a Cu(In,Ga)Se2 film. Particularly, a three-stage process is taught in U.S. Pat. No. 5,441,897 by Noufi. Briefly, the three stages include: deposition to form a thin film of InxSe; addition of copper to the InxSe film to form a Cu-rich CIS film; and addition of InxSe to the Cu-rich CIS film to form a Cu-poor CIS film. It has been shown that a CIS (or CIGS) film provides a more effective absorber in a solar cell when there is less than a one-to-one ratio of the copper to indium (Cu-poor) (or less than a one-to-one ratio of the copper to (indium+gallium) for CIGS) at or near the surface of the absorber that abuts the window or provides the junction with the cadmium sulfide (CdS) or other thin film of the solar cell. For example, high efficiency thin film solar cells may be formed with a Cu-poor region with 22-24.5 at % Cu.
To date, though, solar cell manufacturers have found it difficult to form a Cu-rich CIS or CIGS thin film and then selectively reduce the amount of copper to form a Cu-poor region (e.g., a Cu-poor surface at CIS/CdS junction or interface). Some efforts have been made to utilize copper to substitute for indium in the step of forming a Cu-rich CIS thin film. However, this involves a chemical process or chemical vapor deposition (CVD) requiring very high temperatures (e.g., 1200° C. or the like), which are undesirable in commercial production settings as it increases energy costs and requires significant engineering to provide the high temperature CVD environment.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.
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